Variable inductors are of use in many circuit applications including magnetic amplifiers which vary the inductance of a circuit element to regulate power and resonant circuits which vary the inductance of circuit elements to vary the resonant frequency of the circuit. The simplest way to obtain a variable inductor is by mechanical movement of a connector along an inductive element. However, it is frequently desirable to vary the inductance of a circuit element by means of an electrical signal rather than by mechanical movement.
The saturation effect of magnetic materials may be employed to create a current controlled variable inductor such as that shown in prior art FIG. 1. Variable inductors of this type typically have a limited variation range of 1 to 10 and suffer from parasitic effects such as capacitance and voltage across each series control winding that limit the quality factor of the inductor. Additionally, such current controlled variable inductors of the prior art typically require very high control currents in the range of 0 to 500 mA. FIG. 1 illustrates a current controlled variable inductor of the above-mentioned prior art in which the inductance L.sub.14 of center winding 14 is controlled by the current Ic delivered to outer control windings 12 and 13.
More particularly, FIG. 1 shows a magnetic core 11, consisting of a magnetic material that can be saturated, with three legs 15, 16 and 17. The outer legs 15 and 17 have identical windings 12 and 13 that are connected in series as shown. Control windings 12 and 13 are wound and connected in such a way that the magnetic flux .phi..sub.c in respective legs 15 and 17 of the core arising from the control current Ic through the outer windings 12 and 13 is equal and points in opposite directions. The opposing magnetic flux .phi..sub.c results in cancellation in the center leg 16 of the core. The flux cancellation prevents coupling of AC signals between the center winding 14 and the series control windings 12 and 13. If an AC voltage were applied across the terminals of center winding 14, a voltage would be induced in both of the series windings 12 and 13 but the voltages in the control windings 12 and 13 would be of opposite polarity such that the voltage across the series connection of control windings 12 and 13 would remain zero. The magnetic path for center winding 14 includes outer legs 15 and 17, center legs 16 and the connecting portions 18-21. If the control current Ic through windings 12 and 13 becomes large enough to saturate the legs 15 and 17 of the core, the inductance L.sub.14 of center winding 14 decreases because a portion of the magnetic path for the center winding 14 is saturated. The higher the control current Ic becomes, the lower the inductance L.sub.14 becomes. However, the center leg 16 will not be saturated due to the control current Ic.
The inductance of an inductive circuit element is related to the permeability of the core and the number of turns: ##EQU1##
where L is the inductance of an inductive circuit element;
.mu..sub.0 is the permeability of the magnetic core; PA1 A is the cross-sectional area of the magnetic core; PA1 N is the number of turns of the inductive element; and PA1 l is the length of the inductive element.
In accordance with Equation 1 since the center leg is not saturated, the minimum inductance L.sub.14 is limited by the number of turns and the magnetic permeability of the core material of the center leg 16. Another undesirable side effect of the prior art circuit of FIG. 1 is that the inductance of each of the series connected control windings 12 and 13 changes substantially with a change in the value of the control current Ic. In fact, the inductances of the control windings 12 and 13 change by a greater amount than the inductance of the center winding 14. This condition establishes significant limitations when the prior art variable inductor is part of a regulation loop. The inductor of the prior art circuit FIG. 1 has a limited variation range or requires a very high control current in the order of about 0 to 500 mA. Further, the voltage across each control winding 12 and 13 and the parasitic capacitances of control windings 12 and 13 limit the winding ratio and/or the operating frequency. The inductance of the control windings 12 and 13 changes substantially with the control current Ic.
U.K. Patent 715,610 discloses variable inductive elements having saturable cores. The U.K. '610 variable inductors are illustrated in FIGS. 2A and 2B. The variable inductor of FIG. 2A has series windings on the outer legs of a three leg core, and accordingly is similar to FIG. 1 above. FIG. 2B illustrates parallel windings on the outer legs of a three leg core and a control winding on the center leg of the core. There is no teaching in the '610 U.K. Patent to set the magnetic cross-section of the center leg, relative to the magnetic cross-sections of the outer legs in a variable inductor so that the outer legs and the center leg have substantially equal levels of saturation, in order to obtain a substantially constant inductance of the control winding with a change in current of the control winding. Further, there is no teaching in the '610 U.K. Patent to taper the portions of a three leg core connecting the legs down from the cross-section of the center leg to the cross-sections of the outer legs in order to obtain the largest variation in inductance for a given control current. Further, the '610 U.K. Patent teaches the use of an additional body or additional lamination strips to add cross-sectional area to the center leg of the three leg variable inductor shown in FIG. 2A above. FIG. 7 of the '610 U.K. Patent shows a perspective view of the three leg transductor of FIG. 2A above where additional cross-sectional area of the amount of a x e is added. The additional "bodies" make it difficult if not impossible to maintain a substantially constant inductance of the control winding.
Magnetic amplifiers are known having cores with three legs, parallel windings on the outer legs and a separate winding on the center leg. U.S. Pat. No. 2,229,952 to Whiteley discloses magnetic amplifier embodiments of this type. However, magnetic amplifiers operate in accordance with different principles than variable inductors and have different inputs and outputs. For example, in the magnetic amplifiers by Whiteley mentioned above, the current in the control winding around the center leg biases the core magnetization and does not saturate the core. The core is saturated by the AC signal from the generator 4 and due to the action of diodes 5, only one outer leg is saturated at a time. Each outer leg is saturated at a different time. Each outer leg is alternately, saturated and then not saturated, every cycle of the AC signal. The current through the control winding 2 determines the part of the half cycle during which the core is in saturation. The average DC voltage output relates to the amount of current through the control winding 2. The operation of a magnetic amplifier is to control the DC output voltage in accordance with the control current in control winding 2.